Chain transmissions, in which a chain is engaged with a driving sprocket and one or more driven sprockets, have been widely used as timing transmissions in automobile engines for driving the valve-operating cam or cams from the engine crankshaft.
In these chain transmissions, it is customary to use a standard roller chain or a standard bushing chain, and a standard sprocket. The standard chains and sprockets are defined in the Japanese Industrial Standards (JIS) and in the International Standards (ISO).
Roller chains, rollerless bushing chains, and sprockets, used in chain transmissions are defined in International Standard (ISO 606: 1994(E)) and in Japanese Industrial Standards (JIS B 1801-1997). The International Standard (ISO 606: 1994 (E)) defines tooth forms of chains and sprockets (the “ISO tooth form”), and Japanese Industrial Standards (JIS B 1801-1997) define tooth forms of chains and sprockets (S-tooth forms and U-tooth forms). Both the International Standard (ISO 606: 1994(E)) and the Japanese Industrial Standards (JIS B 1801-1997) are here incorporated by reference. Copies of the relevant parts of both standards are attached. Chain transmissions generally use standard roller chains and standard sprockets, defined in ISO 606: 1994 (E) or JIS B 1801-1997.
As used herein, the term “standard chain” means a chain as defined in International Standard ISO 606: 1994 (E), or in Japanese Industrial Standards JIS B 1801-1997, and the terms “standard sprocket” and “standard tooth form” refer respectively to sprockets and sprocket teeth conforming to the ISO tooth form, or the S-tooth form or U-tooth form according to the above-mentioned Japanese Industrial Standards.
FIGS. 11 and 12 illustrate a chain transmission comprising a sprocket 400 having an ISO tooth form, and a standard roller chain 60. FIG. 12 is an enlarged view of the portion of FIG. 11 labeled “XII”. The ISO tooth forms shown in FIGS. 11 and 12 are defined by the following expressions in ISO 606: 1994(E).d=p/sin(180°/z)df=d−d1dc=df (for a sprocket having an even number of teeth)dc=d cos(90°/z)−d1 (for a sprocket having an odd number of teeth)re(max)=0.12d1(z+2)r1(min)=0.505d1re(min)=0.008d1(z2+180)r1(max)=0.505d1+0.069(d1)1/3 where                p is the chain pitch,        d is the pitch circle diameter,        d1 is the roller outer diameter,        df is the diameter of the tooth gap bottom circle (root diameter),        dc is the caliper diameter of the sprocket        re (max) is the maximum value of the arc of the tooth head,        ri (min) is the minimum value of the radius of the arc of the tooth gap bottom,        re (min) is the minimum value of the arc of the tooth head,        ri (max) is the maximum value of the radius of the        arc of the tooth gap bottom,and        z is the number of sprocket teeth.        
In FIGS. 11 and 12, pa is a chordal pitch of sprocket 400. This chordal pitch pa is equal to the chain pitch p of the standard roller chain 60.
As is apparent from the above expressions, in the standard sprocket 400 shown in FIG. 12, the profile of the tooth gap bottom 43 is in the form of an arc having a radius ri, which is slightly larger than the radius (d1/2) of the roller 62, and the tooth surface 42 is in the form of an arc having a radius re. Tooth surfaces 42 are continuous with the tooth gap bottom portion 43 on both sides of the tooth gap. The diameter df of the tooth gap bottom circle (also referred to as the “root diameter”) is equal to the difference between the pitch circle diameter d and the roller outer diameter d1. Furthermore, the diameter df of the tooth gap bottom circle is substantially the same as the difference between the pitch circle diameter d and twice the radius ri of the arc of the tooth gap bottom.
The standard roller chain is composed of a series of inner and outer links arranged alternately. Each inner link is composed of two inner plates and two bushings. The ends of each bushing are press-fit into bushing holes in the respective inner plates. A roller, having an outer diameter d1 is rotatably fitted on the outer circumference of each bushing. Each outer link is composed of two outer link plates and two connecting pins. The ends of each connecting pin are press-fit into pin holes in the respective outer plates. The outer plates of each link are arranged in overlapping relationship with the inner plates of two inner links, and each pin of an outer link extends through a bushing of an inner link so that the inner and outer links are connected flexibly. FIG. 11 shows only the rollers 62 of the standard roller chain 60, the bushings, inner plates, inner links, connecting pins, outer plates and outer links being omitted. The standard roller chain has a uniform chain pitch p (FIG. 11), which is the distance between the centers of its successive rollers.
In the standard sprocket 400, the tooth gap bottoms and the opposed tooth surfaces 42, which are continuous with the tooth gap bottoms 43, are symmetrical with respect to center lines X of the tooth gap bottoms, each of which connects the rotational center O of the sprocket with the center of a tooth gap bottom 43. The respective center lines X intersect the pitch circle at intersection points a, and a tooth form pitch angle θ is the angle between by adjacent center lines X. Thus the angle θ of the tooth gap bottoms is an angle corresponding to the angular interval between two successive intersection points a on the pitch circle pc. Thus, the tooth form pitch angle θ is determined by the number z of teeth of the sprocket and is defined by the expression θ=360°/z. Furthermore, the tooth form pitch pa is the distance between intersection points a. Therefore, the tooth form pitch pa is a chordal length corresponding to a tooth form pitch angle θ. Since the standard sprocket 400 has uniform tooth form pitch angles θ, the tooth form pitches pa (i.e., the chordal pitches) are arranged uniformly along the circumferential direction of the pitch circle pc. As mentioned previously, the tooth form pitch pa (i.e., the chordal pitch) is equal to the chain pitch p of the standard roller chain 60.
Recent demand for higher power automobile engines, coupled with public consciousness of environmental problems, has led to the development of engines that produce high levels of noise and vibration and to efforts toward reducing that noise and vibration. For example, in a high power engine operating at a high rotational speed, the load on the timing transmission and its contribution to the overall noise produced by the engine become significant. The principal source of timing transmission noise is the engagement sound generated as the chain engages the sprockets.
A measure taken to reduction measures in engagement vibration and noise, is illustrated in FIG. 10, in which a sprocket 400 is provided with an annular elastic member 440 sandwiched between an inner circumferential hub 460 having a keyed a shaft-receiving hole 460a, and an outer circumferential member 420, which includes teeth 420a which are engageable in driving or driven relationship with a roller or bushing chain. Examples of sprockets having sandwiched elastic members are shown in Japanese Laid-Open Utility Model Publication No. Sho. 59-35765, and in Japanese Laid-Open Patent Publication No. Hei. 9-264400.
Since the sprocket 400, having an annular elastic member as shown in FIG. 10, has an ISO tooth form, when the sprocket engages with a standard roller chain 60 as shown in FIG. 11 and the sprocket rotates clockwise, a following roller 62 moves relative to the sprocket in an arc centered on the center 01 of the preceding roller 62 which has been seated on a tooth gap bottom. The arc has the chain pitch p as its radius. Accordingly, the following roller moves in its arcuate path relative to the sprocket, and collides with a tooth gap bottom, near the center thereof, substantially at a right angle. The kinetic energy of the roller is transmitted to the tooth gap bottom without being interfered at the beginning of engagement. Thus, there is a large engagement impact. The performance of the elastic member 440 in shutting out vibration has been found to be insufficient. Moreover, when the impact force is applied to the elastic member 440, the endurance of the elastic member 440 is reduced.
Further, since the chordal tooth form pitch pa of the sprocket 400 is equal to the pitch p of a standard roller chain 60, the respective following rollers 62 abut the teeth of the sprocket 400 at the same abutment position t as shown in FIG. 11. The abutment position in each case is at the point intersection of a center line X and a tooth gap bottom. Therefore, the engagement of a roller or bushing with the 400 is uniformly periodic, and vibration and noise having an order determined by the number of sprocket teeth are increased. The elastic member has been found to be incapable of reducing these noises and vibrations adequately.
The standard roller chain shown in FIG. 11 is a transmitting roller chain defined in the International standard (ISO), and has a uniform chain pitch p (The distance between the centers 01 of the respective rollers 62). A standard bushing chain may be used in place of the standard roller chain 60. In such a case, the elements 62 in FIG. 11 can be regarded as bushings.
Accordingly, an object of the invention is to provide a chain transmission in which a roller of a standard roller chain or a bushing of a standard bushing chain engages with a sprocket tooth, in which the vibration reducing performance of an elastic member incorporated into the sprocket is improved, and in which the endurance of the elastic member is improved.